In the field of optical communication, transceivers using an optical modulator are utilized. Recently, in accordance with demands for miniaturizing optical transmission systems, RF interface connection of an optical modulator mounted in a transceiver module (transponder) also tends to be shortened.
FIG. 1 illustrates a situation where an optical modulator is disposed on an external circuit board 7 configuring a module. As means for realizing shortened connection, as illustrated in FIG. 1, an interface employing a surface mount technology (SMT) using a flexible printed circuit (FPC) 6 and a lead pin 4 has been utilized, progressing from cable connection using a coaxial connector such as a push-on coaxial connector in the related art. Patent Literature No. 1 discloses an optical modulator using a flexible printed circuit.
FIG. 1 illustrates a cross-sectional view of a state where an optical modulator is disposed on an external circuit board. An optical modulator 1 mounts an optical modulation element 2 inside a housing 10 and is hermetically sealed. The reference sign 11 indicates a lid part of the housing. The optical modulation element 2 mounted inside the housing, and the external circuit board 7 are electrically connected to each other through the flexible printed circuit 6 and the lead pin 4 which is disposed in a through-hole (connecting a top surface and a bottom surface of the metal housing in a perpendicular direction) of the housing. In addition, the flexible printed circuit 6 and the lead pin 4 are directly connected to each other. The lead pin 4 and the optical modulation element 2 are subjected to wire bonding (50 and 5) using a gold wire or the like through a relay substrate 3.
In the flexible printed circuit 6, a signal line (signal electrode) and a ground line (ground electrode) made of a conductive material such as Au and Cu are formed on one surface or both surfaces of the circuit made of polyimide or the like. A microstrip line having a wide-width ground electrode formed on one surface and a signal electrode on a strip formed on the other surface is in wide use. In signal lines, there are cases where ground electrodes are also disposed in addition to a signal electrode as in a coplanar line such that the signal electrode is interposed therebetween. Although the flexibility of the flexible printed circuit 6 is degraded, a strip line having multiple layers of ground electrode surfaces or a grounded coplanar line is also used.
When the flexible printed circuit 6 is mounted in the optical modulator 1, a recess portion (spot facing portion) is formed on an external bottom surface of the housing as indicated with a dotted line D in FIG. 1, such that the flexible printed circuit 6 does not protrude from the bottom surface of the optical modulator 1.
However, due to the shape of the lead pin 4, it is difficult for both the flexible printed circuit 6 and the bottom surface (surface A facing the FPC 6) forming the recess portion to be brought into completely tight contact and attached to each other. For example, in a case where the signal line and the ground electrode are configured to be coaxially disposed and a tip of the ground electrode part protrudes from a lower surface of the housing, the FPC 6 and the bottom surface A of the recess portion are separated from each other as much as the protruding height of the lead pin 4. Therefore, a gap S1 is generated between both thereof. More specifically, a gap S1 is generated between an electric wiring part B of the signal electrode, the ground electrode, and the like provided on the FPC 6, and the bottom surface A.
In addition, a gap S2 is generated between the FPC 6 and the external circuit board 7. More specifically, a gap S2 is generated between the electric wiring part B of the signal electrode, the ground electrode, and the like provided on the FPC 6, and an electric wiring part C provided on the external circuit board 7. As a reason therefor, in a case where the lead pin 4 is subjected to solder-fixing on the lower surface side of the FPC 6, the tip of the lead pin 4 protrudes from the lower surface of the FPC 6. In order to prevent this protruding lead pin 4 from coming into contact with the external circuit board 7, the depth of the recess portion is intentionally adjusted such that the gap S2 is generated.
Such gaps S1 and S2 are not necessarily air layers. For example, in a case where an insulating protective film is provided on the front surface (or both surfaces) of the FPC 6, or in a case where an insulating protective film is provided on the front surface of the external circuit board, a gap corresponding to the thickness of the protective film (in this case, a state where the gap is filled with the material of the protective film) is inevitably generated.
A case where the gap S1 or S2 is parallel to the ground electrode surface of the FPC 6 leads to generation of a parallel plate mode as described in FIGS. 2A and 2B. FIGS. 2A and 2B illustrate a state where the housing 10 is disposed on the top surface side of the FPC 6 and the external circuit board 7 (conductive surface of the ground electrode and the like) is disposed on the lower surface side. A microstrip line is configured in the FPC 6 such that a signal electrode 61 is disposed on the top surface of a flexible insulating substrate 60 and a ground electrode 62 is disposed on the lower surface respectively.
FIG. 2A schematically illustrates an electric field of a signal in a strip line-type signal line using dotted line arrows. FIG. 2B schematically illustrates a situation where a part of an electric field which has leaked from the signal line causes a parallel plate mode to be generated between the ground electrode surface of the FPC 6 and the external circuit board 7. The dotted line arrows in the diagram indicate the direction of the electric field. A parallel plate mode is also generated between the ground electrode 62 of the FPC 6 (electric line portion B) and the bottom surface A of the recess portion. In any case where the ground electrode 62 is disposed on one side of the external circuit board 7 and the bottom surface A of the recess portion, if the external circuit board 7 and the bottom surface A of the recess portion are parallel to each other, a parallel plate mode is generated. In a case where the ground electrode 62 is narrow in width and the ratio of the opposing area of the external circuit board 7 and the bottom surface A of the recess portion is significant, a parallel plate mode is generated between the external circuit board 7 and the bottom surface A of the recess portion.
As illustrated in FIG. 2A, a pseudo-TEM mode is generated between the signal electrode 61 and the ground electrode 62. As illustrated in FIG. 2B, in the gap S2, a parallel plate mode is generated between the ground electrode 62 and the external circuit board 7. When such a parallel plate mode is generated, broadband characteristics of a modulation signal applied to the optical modulator deteriorate. Besides, the parallel plate mode is characterized by having no cut-off frequency and is generated no matter how the clearance (gap between the ground electrode 62 and the external circuit board 7), that is, the gap S2 is narrowed. Naturally, a parallel plate mode is also generated in the clearance S1 no matter how the clearance between the top surface A of the recess portion (spot facing portion) of the housing 11 and the FPC 6 is narrowed. For example, in a microwave band or a millimeter wave band, even if the distance is narrowed to the extent of 25 μm such that no cavity resonance takes place, a parallel plate mode is generated between the top surface A and the ground electrode 62 or between a ground electrode and the top surface A in a case where the ground electrode is provided on the top surface of the insulating substrate 60 (not illustrated in FIGS. 2A and 2B).
In addition, a cavity resonance mode corresponding to the clearance, that is, the gap S1 or S2 is also generated in the gap S1 or S2 due to a microwave and a millimeter wave emitted to the space, so that a modulation signal in a particular frequency deteriorates (dips). In order to shorten a wiring distance and to ensure easiness of mounting, the gap S1 or S2 is required to be narrower. In this case, a parallel plate mode is likely to be generated.
As in a DP-BPSK optical modulator, a DQPSK optical modulator, a DP-QPSK optical modulator, and the like, in a case of a broadband optical modulator having a plurality of signal lines disposed on the FPC 6, crosstalk caused between the signal lines through a parallel plate mode leads to a severe problem. Since the crosstalk is caused due to power which has leaked from a signal line and is transferred to another signal line through a parallel plate mode having no cut-off frequency, the crosstalk appears throughout an extremely wide frequency.
In a case of a particular frequency, crosstalk can be reduced by means of a stub circuit or a choke circuit. However, the method is not effective in a case of a broadband modulator having a signal band ranging from a MHz band to a millimeter wave band. As alternative means, crosstalk inside a wiring substrate, for example, crosstalk among a plurality of adjacent microstrip lines can be reduced by means in which a via hole is disposed between the lines, a groove is made in a dielectric substrate between the lines, or the like. However, a parallel plate mode is generated between a ground electrode surface and another ground electrode surface of the wiring substrate regardless of whether a via hole, a groove, or the like is formed inside the substrate. In addition, power which has leaked from a signal line reaches not only an adjacent signal line but also other signal lines, thereby resulting in crosstalk. The same applies not only to a microstrip line but also to a line having a different shape.
In the present invention, a phenomenon of a cavity resonance mode of a microwave and a millimeter wave, a parallel plate mode, or the like is expressed as “a resonance mode or the like”.